Considerable work has been expended in improving techniques for bonding integrated circuit chips to conductive circuit patterns supported on a substrate. One such method uses an anisotropically conductive adhesive between bonding pads on the substrate and matching bonding pads of the integrated circuit chip. The adhesive is typically an insulative polymer containing conductive particles that simultaneously contact the pad of the chip and the pad of the substrate to provide interconnection. The conductive particles do not provide any significant lateral or horizontal conduction; they transmit current only in the vertical direction between substrate and device bonding pads, and so the conduction is referred to as "anisotropic." The adhesive of the polymer is cured after mounting of the chip on the substrate which thereafter provides a permanent structural bond in addition to a conductive interconnection. High component density and bonding pad densities can be accommodated by conductive adhesives, and their use generally reduces assembly costs.
Anisotropic conductive adhesives are particularly promising for reducing the cost of bonding chips having a high density of bonding pads spaced only a very small distance from the bonding pads of the substrate. For example, if they could be used for interconnecting bonding pads spaced only about fifteen microns from substrate bonding pads, with the dimensions of the bonding pads being on the order of only one hundred microns on a side, assembly costs could be considerably reduced compared to other methods, and the losses due to inductance through the connections could be reduced because of the close spacing. Unfortunately, commercially available conductive particles do not have dependably uniform dimensions, and as a result a large-diameter particle may separate matching bonding pads so as to prevent other particles from making conductive interconnections. The occasional large-diameter particle may even prevent adjacent bonding pads from being interconnected which could result in a catastrophic malfunction of an entire system. Additionally, deviations from planarity of the substrate surface may prevent good contact via certain ones of the particles.
The U.S. patent of Tsakagoshi et al., U.S. Pat. No. 4,754,657, granted Apr. 26, 1988, addresses this problem and solves it by providing metal coated polymer balls as the conductive particles. With the metal coating being sufficiently thin, a large-diameter polymer ball will deform under pressure so as to permit contact to be made simultaneously with smaller diameter balls. We have found that a major problem with this technique is that such conductive interconnects are inherently current limited. That is, for the balls to be appropriately deformable, the metal coating on each ball must be appropriately thin, and such thin coatings are incapable of transmitting relatively high current densities. If the conductive coatings on the polymer balls are made thick enough to conduct high current densities, we have found that they prevent deformation of the ball, and the technique for ensuring reliability does not work. The thin coatings are also more inductive than solid metal particles, which reduces the speed at which the interconnected circuits can be operated.
Accordingly, there has been a long-felt need for conductive adhesive techniques that can provide reliable conduction and bonding, and particularly for such techniques that are consistent with relatively high current densities and/or low inductance.